Boulder is a town in and the county seat of Jefferson County, Montana, United States. It is on the north bank of the Boulder River between Butte and Helena, slightly east of the Continental Divide, at the intersection of Interstate 15 and Montana Highway 69. The population was 1,201 at the 2020 census. It is part of the Helena Micropolitan Statistical Area, which includes all of Lewis and Clark and Jefferson counties; its population is 83,058 according to the 2020 Census.

In the U.S. state of Montana, Interstate 15 continues onward from Idaho for nearly 400 miles (640 km) through the cities of Butte, Helena, and Great Falls, intersecting with I-90, I-115, and I-315. I-15 reaches its northern terminus at the international border with Alberta, Canada.